Memory devices such as dynamic random access memories (DRAM) typically perform sequential operations such that certain operations cannot be initiated until a prior event has been completed. To insure that a prior event was completed, a "time-out" circuit could be used to delay the initiation of a subsequent event. In operation the start of a first event activates a time-out circuit, after a predetermined time the time-out circuit enables the start of a second event. While this type of circuit fulfills some of the needs of the memory device, a worst case time-out must be used to insure that all variations in voltage, temperature and fabrication have been accounted for. Thus, an undesirable wait time is often experienced between the termination of the first event and the start of the subsequent second event.
Tracking circuitry can be included in the memory devices to allow the memory devices to operate more efficiently by reducing wait time experienced as a result of time-out circuits. The tracking circuitry monitors internal signals to determine when an event has occurred. For example, DRAM memory cells are typically arranged in an array having rows and columns. Data is read from the memory by activating a row, referred to as a word line, which couples memory cells corresponding to that row to digit lines which define columns of the array. Internal circuitry such as a sense amplifier is used to detect voltage changes on the digit lines. It will be appreciated that the sense amplifier circuitry should not be activated until a memory cell has been coupled to the digit line. To insure that a memory cell has been fully coupled to a digit line, the word line signal is monitored by a tracking circuit.
FIG. 1a illustrates a word line tracking circuit 5 which uses an inverter 6 having an input (Vin) connected to receive a word line activation signal. The output of the inverter is connected to a switching circuit 7 through a coupling line designed to simulate the resistance and capacitance characteristics of a word line. The tracking circuit 8 of FIG. 1b includes an inverter and a switching circuit which switches when the input is approximately 1/2 Vcc. The out-put of the switching circuits of FIGS. 1a and 1b are used to initiate an operation performed during a memory read. The tracking circuits of FIGS. 1a and 1b do not accurately track variations in voltage, temperature, or process conditions which occur during fabrication and operation of the memory devices.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for tracking circuitry which accurately tracks word line signals over a wide range of variables.